Biggest cause of a vacuum leak in an FE engine is the intake gasket and it won't show up outside the engine. The gasket surface is under the valve cover. The heavy intake is difficult to place. I glued the gaskets to the head with 3M weatherstrip adhesive so it wouldn't move.
I would say you have a vacuum leak. Do you have a hand held propane torch ? Put a rubber hose on the tip and turn the propane on. Disclaimer , don't light the torch. Go around the base of the carb and around the ports of the intake. If the idle speeds up you have found your leak. Much better than starting fluid and easier on the motor.
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